Investment casting (or the lost wax process) is one of several techniques for manufacturing items such as jewelry, ornaments, figurines and machine parts. For example, to make a jewelry finger ring using the lost wax process, a jeweler may first create a ring design to serve as a model. The model may be created of metal, foam, wax, clay, plastic, wood or other shapeable material. A gate (also called a runner or sprue branch) may be attached to each model. A mold may be created of the model, and may comprise a substantially exact negative of the model. The mold may be created of any suitable flexible material, such as natural or synthetic rubber, or polyurethane. The mold may be cut away from the model so as to leave the mold in one or more pieces. The mold may be provided with vents to allow air to escape during wax injection. Plastic may be used instead of wax. The mold may then be placed in a mold frame and injected with wax to create a wax pattern of the model. After solidifying, the wax pattern may be removed from the mold. Multiple wax patterns may be created. The gated patterns may be attached to a central sprue rod to form a tree-like structure (“tree”). Thus assembled, the tree may then be invested in a ceramic, plaster or other refractory material and placed in an oven for high-temperature curing. As it cures, the investment material hardens into a durable foundry mold. The heat of the oven melts or burns the wax out of the mold, leaving channels in the mold into which castable metal alloy may flow. Castable metal alloys, such as sterling silver, may be melted, and then poured into the investment mold and allowed to solidify. After the alloy solidifies and cools, the mold may be broken or split open to allow removal of a tree of castings, each of which may be substantially identical to the model. The rings may be cut from their sprue branches and finished. The finishing process may include removing sprue stumps, mold seams or other imperfections, and polishing.
The model ring may be created in a single ring size. The foregoing process may be accomplished to create patterns for each ring design that are then sized for different finger sizes. Generally, twelve to fourteen patterns may be made in whole and half sizes. Each pattern may then be used to create a metal model of each size (“size model”). Each size model may then be used to create multiple finished rings as described above. For example, a sprue may be soldered to a size model, finished to remove imperfections, and may be plated with, for example, rhodium, gold or nickel to protect the model during the high-temperature vulcanization process.
To fabricate a mold, a mold technician may pack the size model with, e.g., up to twelve layers of ⅛″ un-vulcanized rubber sheet, and may install a sprue former on the end of the sprue. The sprue former may comprise a conical piece of brass that fits the nozzle of a wax or plastic injector. The packed size model may then be placed in a steel or aluminum frame. The assembly may then be placed in a vulcanizing apparatus and the rubber vulcanized under pressure at approximately 300° F. for approximately seven minutes per rubber layer. The inert metal plating may protect the size model from degradation during the vulcanization process.
After the mold is removed from the vulcanizing apparatus and cools, the mold technician may then cut the rubber mold away from the size model with a scalpel. The mold cutting process typically requires high skill. Typically, a flap may be cut in the rubber mold to allow removal of the ring, or the mold may be cut into two or more pieces indexed to allow accurate reassembly. The mold may be vented to allow air to escape during injection of pattern material. Injection of pattern material and casting may be accomplished as described above.
All models must be appropriately dimensioned with regard to their internal geometry to obtain the desired sizes. This requires knowledge of the degree of shrinkage of wax or plastic patterns occurring during process steps. For ring models, the internal circumference of the rings must be appropriately dimensioned to obtain the desired full and half sizes. To estimate shrinkage, the patterns of the largest and smallest ring sizes may be molded. The degree of shrinkage for these patterns may then be measured. Based on these measurements, shrinkage rates for the other sizes may be estimated. For example, a ring size model may be created approximately ¼ ring size larger than the desired final dimensions in order to accommodate mold-related shrinkage.
Generally, the quality of the finished rings may depend on, among other things, the mold materials used, the skill and cutting technique of the mold technician, the wax or plastic injection equipment and settings, the investment equipment and materials, and the casting equipment and materials. For example, deformation of the mold can result in too much variance in a ring's internal dimension or size for a given size model. Or, for example, the mold frame and injection nozzle may distort the mold. There exists a need for a method and apparatus for fabricating injection molds and using such molds to produce patterns of more consistent quality so as to improve the quality of the final manufactured component.